I know someone that actually recently gave notice that has a very similar position as yourself, albeit it's for a staff of ~100 but for two distinct locations:
- Systems administration:: Manage over 20 virtual machines: couple of windows for AD and WSUS but mostly Linux (which the admin also setup from scratch), backup solution with Veeam
- Network administration:: from Site-site VPN to VLAN to fail-over connection to cabling.
- Software development:: Develop and maintain in-house software solutions and integrate with other software (this includes database setup, management of Firebird, PostgreSQL, mySQL and a couple I cannot recall the names of but less common)
- Technical support:: Provide support for the developed software and end user support
This person is an invaluable key asset and I know there is another person that has been working (source) with the whole environment for > 5 years and still does not know the level of depth the current admin has.
Personally, I think this admin should be part-owner or shareholder as this person also develops the software that saves the business money and automates a lot of processes, But I do not know the agreement and the decision pieces behind it.
If your job responsibilities has, as such changed so significantly and this was not reflected by your salary, you should consider speaking to your manager. Highlight the additional responsibilities that were not part of the hiring agreement.